A polycarbonate-based resin is a polymer excellent in transparency, heat resistance, and impact resistance and is widely used at present as an engineering plastic in the industrial field.
As a method of producing the polycarbonate-based resin, a method involving allowing an aromatic dihydroxy compound such as bisphenol A, and phosgene to react directly with each other (interfacial polymerization method) is known as a method of producing a high-quality polycarbonate.
As a method of industrially producing a polycarbonate by interfacial polymerization method, there is adopted a method involving bubbling phosgene into an alkali aqueous solution of a bisphenol to form a polycarbonate oligomer having a reactive chloroformate group, and further allowing a condensation reaction (polymerization reaction) of the polycarbonate oligomer and the bisphenol to proceed in the presence of a polymerization catalyst such as a tertiary amine, and an alkali aqueous solution simultaneously with or successively after the formation of the polycarbonate oligomer. From the viewpoints of solubility, handling properties, and the like, methylene chloride is mainly used as a water-insoluble organic solvent on an industrial scale.
The methylene chloride used in the reaction step is generally recovered and then reused for the reaction step (see Patent Document 1). In addition, waste water after the polymerization reaction, waste water formed after a washing step, and waste water formed after a granulation step contain an inorganic material such as sodium chloride and an organic material such as a phenol and a polycarbonate. In order to remove such organic material from the aqueous phase and clean the waste water, the organic material is extracted and removed from the waste water by using an organic solvent, preferably the same water-insoluble organic solvent as that used in the polycarbonate production step, such as methylene chloride. The water-insoluble organic solvent containing the extracted and removed phenol and polymer is reused for the polymerization reaction step (see Patent Document 2).
In addition, in production of phosgene in the method of producing the polycarbonate-based resin, carbon tetrachloride (CCl4) is produced as a by-product and carbon tetrachloride is gradually accumulated in a reactor by cyclic use of methylene chloride. Accordingly, a concentration of carbon tetrachloride in the polycarbonate-based resin to be produced increases. The concentration of carbon tetrachloride in the polycarbonate-based resin is preferably suppressed to a low level because carbon tetrachloride is responsible for deterioration in color tone of the polycarbonate-based resin and corrosion of a die. As a method of suppressing the concentration of carbon tetrachloride in the polycarbonate-based resin to a low level, there has been known a method involving producing phosgene with a catalyst layer diluted with a material substantially inert to carbon monoxide and chlorine to reduce an amount of carbon tetrachloride to be produced as a by-product at the time of the production of phosgene (see Patent Document 3), or a method involving purifying circulating methylene chloride by distillation in the production of the polycarbonate-based resin (see Patent Document 4).
It should be noted that, among the polycarbonate-based resins, a polycarbonate-polyorganosiloxane polymer (hereinafter sometimes referred to as “PC-POS”) has been attracting attention because of its high impact resistance, high chemical resistance, and high flame retardancy, and the polymer has been expected to find utilization in a wide variety of fields such as the field of electrical and electronic equipment and the field of an automobile. As a method of producing the PC-POS, there is known a method involving allowing a dihydric phenol-based compound and phosgene to react with each other to produce a polycarbonate oligomer, and polymerizing the polycarbonate oligomer with a polyorganosiloxane in the presence of methylene chloride, an alkaline compound aqueous solution, a dihydric phenol-based compound, and a polymerization catalyst (see Patent Document 5).